Composite Tube Having Co-Bonded End Fittings

ABSTRACT

A structural member such as a strut includes a composite material tube having metal end fittings that are attached to the tube by co-bonded, double shear joints. The double shear bond joint construction reduces the residual stress on the bonds that result from mismatch of the coefficients of thermal expansion of the composite tube and the metal end fittings. The ends of the fittings that are bonded to the tube may include a stepped profile that functions to limit the peak stresses in the bonds.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/747,389, filed May 11, 2007, status pending, which is acontinuation-in-part application of U.S. patent application Ser. No.11/607,546, filed Dec. 2, 2006, status pending.

TECHNICAL FIELD

This disclosure broadly relates to composite structural members, anddeals more particularly with a composite tube having co-bonded metal endfittings.

BACKGROUND

Structural members formed from both composite and metallic materials areused in a variety of applications in the aerospace industry. Forexample, structural members such as struts may be formed from acomposite material tube having metallic end fittings that attach thestrut to other structure in an aerospace vehicle, such as a commercialaircraft. The strut may act as either a support or a connecting member,transferring force in either direction along the longitudinal axis ofthe strut. Thus, the strut may be subjected to either compressive ortension loading. The use of a composite tube normally provides a weightadvantage over a metallic tube, while the use of metallic end fittingsprovides additional strength at points of attachment.

In some cases, the metallic end fittings may be attached to thecomposite tube using fasteners that pass through the tube and thefitting. This attachment technique may result in stress concentrationsin the tube in the area around the fasteners, and therefore requiresthat the tube have a greater thickness in order to accommodate theselocalized stresses. This additional tube thickness increases both theweight of the structural member, and the cost of materials.

The use of fasteners may be obviated by bonding the end fittingsdirectly to the composite tube. In order to form the attachment bond, acylindrical section of the end fitting may be inserted into an open endof the tube and a bond is formed at the overlapping, contacting areasbetween the interior wall of the tube and the exterior wall of the endfitting. The axial length of the bond must be sufficient to withstandshear forces produced by the compression and/or tension loads which thestructural member is designed to transfer. Higher loading thereforerequires a longer bond length between the end fitting and the tube.Longer bond lengths create a problem, however, due to the difference inthe coefficients of thermal expansion (CTE) of the composite tubecompared to metal end fittings. This problem is due, in part to theprocess used to produce the bond. The bonding process involves curingthe composite materials forming the tube at elevated temperature whilethe metal fitting is attached to the tube. In some cases, the metalfitting may be bonded to a prefabricated tube. In either case, the metalfitting expands a greater amount than the tube during the curingprocess, since the CTE of metal is higher than that of the compositematerial. Subsequent cooling of the metal and composite material resultsin the metal and the composite material contracting at different rates,producing residual stresses in the bond area. The residual stresses maybe exacerbated as a result of the bond being subjected to thermalcycling and tension and/or compression loading during in-flight service.Thermal cycling may occur during typical aircraft operations whenaircraft components are exposed to temperatures of about 90° F. or moreon the ground to as low as about −60° F. or lower at typical flightaltitudes.

Accordingly, there is a need for a bond construction that overcomes theproblems mentioned above. Embodiments of the disclosure are directedtoward satisfying this need.

SUMMARY

According to one embodiment of the disclosure, a structural member mayinclude: a composite material tube having co-bonded inner and outer tubewall portions; and, a metal fitting having at least a section disposedbetween and co-bonded to the inner and outer tube wall portions. Thesection of the fitting forms a first bond joint with the inner tube wallportion and a second bond joint with the outer tube wall portion,providing a double shear bond. In one embodiment, the bond joints may bescarf joints, while in another embodiment, the joint may have steps ofdecreasing thickness in an axial direction. The double shear bond jointmay reduce stress on the bond resulting from the mismatch of thecoefficients of thermal expansion of the metal fitting and the compositetube. Co-bonding of the fitting with the composite tube results in abond strength that may satisfy design load requirements, without theneed for fasteners, although fasteners may also be used.

The co-bonded double shear joint of at least one embodiment may alsoreduce the residual stresses present in the bond to acceptable levels,and may also reduce peel stresses in the joint, especially at the endsof the joint. The double shear joint construction is also advantageousin that the eccentricity of the components forming the joint may bereduced.

According to another disclosed embodiment, an aircraft strut isprovided, comprising: a tubular member formed of laminated plies ofreinforced polymer resin; at least one metal fitting; and twooverlapping, co-bonded joints between the tubular member and the metalfitting. The tubular member may include an inner tube wall portion andan outer tube wall portion, and the metal fitting may include a tangdisposed between and co-bonded to the inner and outer tube wallportions. The tang may be disposed coaxial with the tubular member andmay be tapered in the direction of the length of the tubular member. Thetang may include a plurality of stepped wall surfaces, and the inner andouter tube wall portions each may include multiple plies of thereinforced polymer resin co-bonded to each of the steps.

According to another disclosed embodiment, a composite material strut isprovided, comprising: a tube having a wall including laminated plies offiber reinforced resin, and a metal fitting having a tapered portionextending into an end of the tube wall. The tapered portion extendsbetween and is co-bonded to the plies in the tube wall to form a double,overlapping joint between the tube and the fitting. The laminated pliesmay be arranged in groups forming ply drop offs along the length of thetube in the area of the double joint.

These and further features, aspects and advantages of the embodimentswill become better understood with reference to the followingillustrations, description and claims.

BRIEF DESCRIPTION OF THE ILLUSTRATIONS

FIG. 1 is a perspective illustration of a strut having a compositematerial tube and metallic end fittings, according to an embodiment.

FIG. 2 is a longitudinal section illustration of an end of the strutdepicted in FIG. 1, showing the use of a scarf joint according to oneembodiment.

FIG. 3 is a perspective illustration of one end of the strut shown inFIG. 1, a portion of the outer tube wall portion having been broken awayto reveal the inner tube wall.

FIG. 4 is a longitudinal sectional illustration taken through the end ofthe strut shown in FIG. 1, but depicting a stepped bond joint whichforms another embodiment.

FIG. 4A is a fragmentary, longitudinal section illustration takenthrough the end of the strut shown in FIG. 1, but showing an alternatelay-up arrangement.

FIG. 4B is a fragmentary, longitudinal section illustration takenthrough the end of the strut shown in FIG. 1, but showing another lay-uparrangement.

FIG. 5 is an enlarged illustration of a section of the stepped bondjoint shown in FIG. 4, designated as “A”.

FIG. 6 is an end illustration of the end fitting shown in FIG. 3.

FIG. 7 is a sectional illustration taken along the line 7-7 in FIG. 6.

FIG. 8 is a plan illustration of the end fitting shown in FIG. 6.

FIG. 9 is a side illustration of the end fitting shown in FIG. 6.

FIG. 10 is a diagrammatic illustration of a ply layup schedule for anembodiment.

FIG. 11 is a cross sectional illustration of an alternate form of atang.

FIG. 12 is a side illustration of a mandrel rod having an expandablemandrel shown in a deflated condition.

FIG. 13 is a longitudinal sectional illustration of a female mandrelmold into which mandrel rod depicted in FIG. 10 has been inserted.

FIG. 14 is an illustration similar to FIG. 11, but showing the mandrelhaving been inflated.

FIG. 15 is a side illustration of the mandrel wrapped with multipleplies of fiber reinforced material to form an inner tube wall portion.

FIG. 16 is an illustration similar to FIG. 13, but showing the firstlay-up having been debulked and end fittings having been installed overthe inner tube wall portion.

FIG. 17 is an illustration similar to FIG. 14 but showing the firstlay-up having been placed in a lay-up mold for compaction and curing.

FIG. 18 a side illustration showing the second lay-up having beenapplied over sections of the end fittings and the first lay-up to forman outer tube wall portion.

FIG. 19 is a simplified flow diagram showing the steps for making thecomposite tube having co-bonded end fittings according to an embodiment.

DETAILED DESCRIPTION

Referring first to FIGS. 1 and 2, a structural member in the form of astrut 20 may comprise a cylindrical tube 22 and a pair of end fittings24 secured to the opposite ends of tube 22 by double shear bonds. Thetube 22 may comprise, but is not limited to a composite material, suchas multiple laminated plies of a fiber reinforced polymer resin. Anexample of multiple plies of a fiber reinforced polymer resin may becarbon fiber reinforced epoxy. The tube 22 may include an inner tubewall portion 32, and an outer tube wall portion 34 which are co-bonded,as shown in FIG. 2 as a cylinder. Cylindrical tube 22 may have othercross sectional shapes such as, but not limited to square, triangle,hexagon, or pentagon.

Each of the end fittings 24 may be, but is not limited to a metal suchas aluminum or titanium, or a composite end fitting. A metallic endfitting may be formed by casting, machining or other commonmanufacturing techniques. A composite end fitting may include metallicinserts and/or metallic bushings. Each of the end fittings 24 mayinclude a clevis 30 provided with aligned openings 26. While a doubletab 31 configuration is shown, a single tab or triple tab configurationor more than 3 tab configurations are within the scope of theembodiments of the disclosure. The openings 26 may allow the strut 20 tobe connected by pins (not shown) or other pivoting and/or fasteningmeans to structural components, such as in an aircraft.

Depending upon the particular application, strut 20 may function totransfer axial loads bi-directionally, so that the strut 20 may beeither placed in tension or compression, or both in alternating fashion,along its central axis. Each of the end fittings 24 may include an axialopening 28 that is aligned with the central axis of the tube 22 forpurposes which will become apparent later.

As best seen in FIG. 2, each of the end fittings 24 may include aninterior area 35 that is generally hollow in order to reduce the weightof the end fitting 24, and a generally cylindrical open end defining atapered, cylindrical tang 36. The tang 36 may have a tapered crosssection that is disposed between and co-bonded to the inner and outertube wall portions 32, 34, respectively. As will be discussed later, theinner and outer tube wall portions 32, 34, may be formed from laminateshaving tapered profiles that complementally match the tapered crosssection of the tang 36 so as to define an overlapping, double scarfjoint 37. The inner and outer tube wall portions 32, 34, respectivelyform, in combination with the tang 36, an overlapping, double shear bondat the double scarf joint 37.

While not shown, a coupling means, such as, but not limited to afastener may couple wall portions 32 and 34 to the tang 36. A couplingmeans may work with co-bonding or singularly without co-bonding.

Reference is now made to FIGS. 3-9 which depict an alternateconstruction of the composite tube 22 having co-bonded end fittings 24.The tang 36 on each of the end fittings 24 may be provided with aplurality of inner and outer steps 38 such that the outside diameter ofthe tang 36 progressively decreases in the direction away from axialopening 28, while the inside diameter of the tang 36 increases. Thewalls of each of the steps 38 are cylindrical.

As can be seen in FIGS. 4 and 5, the inner and outer tube wall portions32, 34 each may comprise a plurality of plies of composite material,such as, but not limited to a fiber reinforced polymer resin which maybe fabricated using techniques described later below. The laminatedplies 42 (FIG. 5) may be arranged in groups 40 having progressivelygreater lengths in the direction of the end fitting 24. Each ply group40 terminates at an end of one of the steps 38, so that the plies 42 areeffectively tailored in their lengths to complementally match theprofile of the steps 38. The plies 42 are layed up to form the inner andouter tube wall portions 32, 34 which may be co-bonded along with thetang 36 to form a stepped, double shear bond joint 39. The use of thesteps 38 may effectively divide the total amount of the residual stressin the resulting bond so that these stresses peak at each step 38. Insome applications, the stepped, double shear bond joint 39 shown in FIG.4 may be preferable to the double scarf joint 37 described in connectionwith FIG. 2.

FIG. 10 shows one particular ply build up that may be suitable for usein a strut 20 used to support, for example, an engine and/or pylon (notshown) on a wing of an aircraft. The number and orientation of the pliesin each group 40 forming the outer tube wall portion 34 are shown in aply schedule 45. For example, the outermost ply group 40 a comprises 4individual plies having reinforcing fibers respectively oriented at 45,90, −45 and 0 degrees, relative to an orientation reference axis. Thenumber of plies in each group 40 will depend on the depth of each of thesteps on the tang 36, as well as the thickness of the individual plies.

FIG. 11 illustrates an alternate form of the tang 36. The wall 51 ofeach step 38 on the inside face of the tang 36 is cylindrical in shape.However, the wall 53 of each of the steps on the outside face of thetang 36 are conical in shape, resulting in an inwardly tapered wallprofile. Tapering of walls 53 reduces the amount of material in the tang36, thereby reducing the weight of the fitting 24. Alternatively, thewalls 51 may be tapered and the walls 53 may be cylindrical.

Attention is directed to FIG. 19 along with FIGS. 12-18 which depict thesteps in making the composite tube 22 having co-bonded end fittings 24described above in connection with FIGS. 1-11. As shown in FIG. 12, amandrel rod 44 is provided with an flexible mandrel 46 that maycircumscribe mandrel rod 44. In the illustrated example, the flexiblemandrel 46 may comprise a flexible, inflatable bladder. Mandrel rod 44may include a pair of indexing marks 48 on opposite ends thereof, forpurposes that will become apparent later.

Beginning with step 56, the mandrel rod 44 may be axially inserted intoa female bladder mold 50, as shown in FIG. 13, which has an interiorcavity wall 52 corresponding to the desired shape of a mandrel to beformed. The female bladder mold 50 may then be evacuated, causing theflexible mandrel 46 to expand within the cavity. Next, at step 58, theflexible mandrel 46 may be filled with a granular material such as, butnot limited to sand or ceramic beads. A pressurized source of thegranular material may be connected to an axial conduit (not shown)within the mandrel rod 44, which in turn is connected with the interiorof the flexible mandrel 46.

Next, at step 62, the flexible mandrel 46 may be sealed and evacuated toform a partial vacuum. This partial vacuum may compress the flexiblemandrel 46 against the granulated filler material so as to make itsomewhat rigid and assume the desired mandrel shape. It should be notedhere that other types of constructions could be used to form theflexible mandrel 46. For example, an expandable metal or break-downmandrel (not shown) could be employed for ply lay-up rather than theflexible bladder illustrated in the drawings. The flexible mandrel 46 orother known, internal bagging material may then be used during lay-upand/or for curing of the inner lay-up 41.

At step 64, multiple hoop plies of a composite material may be appliedto the flexible mandrel 46, as shown in FIG. 13, resulting in theformation of a first, inner lay-up 41 that may define the inner tubewall portion, such as inner tube wall portion 32 of FIG. 2. The pliesforming inner lay-up 41 may comprise, for example, successive, uncuredlayers of carbon reinforced epoxy material in the form of sheets or atape in which the orientation direction of the reinforcing fiberalternates according to known ply orientation schemes. The inner lay-up41 may be formed by wrapping each of the hoop plies one revolution (360degrees) or less around the flexible mandrel 46. In other words, wrapeach hoop ply of the inner lay-up around the flexible mandrel 46 onlyonce or less. By avoiding plies that wrap more than one revolution, thereinforcing fibers are allowed to move radially during subsequentcompaction of the inner lay-up 41.

At step 66, the inner lay-up 41 may be debulked to remove excess airfrom the lay-up plies and thereby better consolidate the plies. Thedebulking process may be carried out within a vacuum bag (not shown)using vacuum pressure.

Next, at step 68 the end fittings 24 are installed over the inner lay-up41. This step is carried out by passing the end fittings 24 over theends of the mandrel rod 44, allowing the mandrel rod 44 to pass throughthe axial openings 28 in the end fittings 24. The tang 36 of the endfittings 24 are sleeved over the inner lay-up 41. As previouslyindicated, the lengths of the plies forming the inner lay-up 41 may betailored so as to either match the tapered cross section of the tang 36of the end fitting 24 shown in FIG. 2, or the steps 38 of the endfitting 24 shown in FIGS. 4 and 5. As the end fittings 24 are installedover the outer ends of the inner lay-up 41, the indexing marks 48 may beused to align the end fittings 24 relative to each other so that theopenings 26 in the clevis of the two fittings 24 are in a desiredrotational position relative to each other.

At step 70, a female mold 54 may be placed over the inner lay-up 41 andthe tang 36, as can be seen in FIG. 15. The female mold 54 may beevacuated, creating a partial vacuum that draws flexible mandrel 46shown in FIG. 14 and the plies in the inner lay-up 41 into contact withthe interior walls of the female mold 54 shown in FIG. 17 therebycompacting the plies. The female mold 54 may be placed in an autoclaveand heated to the necessary temperature in order to cure the innerlay-up either during or after the compaction process.

Next, the female mold 54 may be removed at step 74. At this point, theinner lay-up 41 defining the inner tube wall portion 32 may be fullycompacted and cured, and may be co-bonded to the inside face of the tang36 of end fitting 24. Then, at step 76, the flexible mandrel 46 may bedeflated and the mandrel rod 44 is removed from the cylindrical tube 22.

At step 78, multiple, uncured plies of composite material may be appliedover the inner lay-up as well as over tangs 36 to form a second, outerlay-up 43 that defines the outer tube wall portion 34 of FIG. 2. Theplies in the outer lay-up 43 may be similar or dissimilar to those usedin the inner lay-up, comprising, for example, carbon fiber reinforcedepoxy resin, in which the plies are arranged in alternating layers ofmultiple fiber orientations (e.g. +45/0/90). Other ply orientations maybe used. The plies in the outer lay-up 43 may be wrapped one or moretimes around the inner lay-up 41. Like the inner lay-up 41, the plies inthe outer lay-up 43 may be tailored in length so as to conform to eitherthe profile of the unstepped tapered tang 36 shown in FIG. 2, or thestepped tang 36 shown in FIGS. 4 and 5. It should be noted here that thenumber of piles used to form the inner and outer lay-ups 41, 43respectively may vary depending on the particular application andperformance requirements. In one embodiment for example, a build up ofthirty three plies was found to be satisfactory for the inner lay-up 41and thirty three plies on the outer lay-up 43 as well.

It may also be possible for an inner lay-up 41 or an outer lay-up 43 tonot extend the entire length of cylindrical tube 22. As shown in FIGS.4A and 4B, inner lay-up 41 or outer lay-up 43 may taper over a bond toouter lay-up 43 or inner lay-up 41, respectively. Tapering sections onboth tube ends may form a double butted cylindrical tube 22. In anotherembodiment, a single butted tube may be formed.

At step 80, the outer lay-up 43 may be subjected to compaction andcuring using conventional techniques. For example, the cylindrical tube22 may be vacuum bagged with the vacuum bag being evacuated and placedin an autoclave (not shown) at elevated temperature until the outerlay-up 43 may be fully compacted and cured. As a result of thiscompaction and curing process, the outer lay-up 43 forming the outertube wall portion 34 is co-bonded with the inner tube wall portion 32and with the outer face of the tang 36 on the end fittings 24.

Although the embodiments of this disclosure have been described withrespect to certain exemplary embodiments, it is to be understood thatthe specific embodiments are for purposes of illustration and notlimitation, as other variations will occur to those of skill in the art.

1. A structural member, comprising: a composite material tube, thecomposite material tube comprising a first cured inner tube and a secondcured outer tube, wherein the first cured inner tube and the secondcured outer tube are co-bonded along a respective wall of the firstcured inner tube and the second cured outer tube; and, a metal fitting,wherein a portion of the metal fitting is disposed between and co-bondedto the first cured inner tube and the second cured outer tube, whereinan intersection between the metal fitting and the first cured inner tubecomprises a first bond joint, and wherein an intersection between themetal fitting and the second cured outer tube comprises a second bondjoint, at least one of the first and second bond joints comprising astepped joint. 2-3. (canceled)
 4. The structural member of claim 1,wherein the metal fitting comprises a cylindrical section, and wherein athickness of the cylindrical section tapers in a direction toward thecomposite material tube.
 5. The structural member of claim 1, whereinthe first cured inner tube and the second cured outer tube each comprisemultiple plies of composite material.
 6. The structural member of claim1, wherein one of the first cured inner tube and the second cured outertube extends only a portion of the length of the composite materialtube. 7-8. (canceled)
 9. The structural member of claim 1, wherein: themetal fitting includes a cylindrical section having a tapered thickness,and, the first cured inner tube and the second cured outer tube eachhave a taper complementing the tapered thickness of the cylindricalsection of the metal fitting.
 10. The structural member of claim 9,wherein the taper of each of the first cured inner tube and the secondcured outer tube is defined by plies increasing in length in thedirection of the metal fitting.
 11. The structural member of claim 4,wherein the thickness of the cylindrical section of the metal fittingcomprises steps defining differing thicknesses. 12-18. (canceled)
 19. Astructural member, comprising: a composite material tube, the compositematerial tube comprising plies, the plies forming a first cured innertube and a second cured outer tube, wherein the first cured inner tubeand the second cured outer tube are co-bonded along a respective wall ofthe first cured inner tube and the second cured outer tube; and, a metalfitting having a tapered portion, the tapered portion disposed betweenand co-bonded to the first cured inner tube and the second cured outertube to form a joint between the composite material tube and the metalfitting.
 20. The structural member of claim 19, wherein the plies arearranged in groups forming ply drop offs along the length of thecomposite material tube in the area of the joint.
 21. The structuralmember of claim 20, wherein: the tapered portion of the metal fittingcomprises a plurality of stepped circular walls of successively smallerdiameter in the direction of the tube, and the groups of plies arerespectively co-bonded to the circular walls.
 22. The structural memberof claim 21, wherein at least some of the walls are tapered in adirection opposite to the direction of the taper of the tapered portion.23. The structural member of claim 19, wherein: the tapered portion ofthe fitting includes first and second sets of oppositely facing stepsalong the length of the tapered portion, and wherein the plies arearranged in groups respectively received within the steps.
 24. Thestructural member of claim 23, wherein each of the steps in the firstand second sets thereof extend circumferentially completely around thetapered portion of the fitting.
 25. The structural member of claim 1,wherein each of the first and second bond joints comprise a steppedshear joint.
 26. The structural member of claim 1, wherein the metalfitting comprises a cylindrical section, wherein a thickness of thecylindrical section decreases in a direction toward the compositematerial tube, and wherein the thickness of the cylindrical sectiondecreases through a plurality of steps.
 27. The structural member ofclaim 1, wherein each of the first cured inner tube and the second curedouter tube comprises a different structural layup.
 28. The structuralmember of claim 1, wherein each of the first cured inner tube and thesecond cured outer tube comprises a different number of plies.
 29. Thestructural member of claim 1, wherein the metal fitting comprises acylindrical section, wherein the cylindrical section comprises a thirdinner wall and a fourth outer wall, wherein each of the third inner walland the fourth outer wall are stepped, and wherein steps of at least oneof the third inner wall or the fourth outer wall are tapered.
 30. Thestructural member of claim 29, wherein tapered steps of at least one ofthe third inner wall or the fourth outer wall taper in a direction awayfrom the composite material tube.
 31. The structural member of claim 1,wherein the metal fitting comprises a hollow interior, and acircumferentially extending wall surrounding the hollow interior. 32.The structural member of claim 1, wherein the metal fitting is co-bondedto the first cured inner tube and the second cured outer tube exclusiveof a separate non-integral layer of adhesive.
 33. The structural memberof claim 1, wherein the first cured inner tube comprises at least onehoop ply, the at least one hoop ply extending less than fullcircumference of the first cured inner tube.
 34. The structural memberof claim 19, wherein the first cured inner tube comprises at least onehoop ply, the at least one hoop ply extending less than fullcircumference of the first cured inner tube.
 35. A structural member,comprising: a composite material tube, the composite material tubecomprising a first cured inner tube and a second cured outer tube,wherein the first cured inner tube and the second cured outer tube areco-bonded along a respective wall of the first cured inner tube and thesecond cured outer tube, wherein the first cured inner tube comprises atleast one hoop ply, the at least one hoop ply extending less than fullcircumference of the first cured inner tube; and, a metal fitting,wherein a portion of the metal fitting is disposed between and co-bondedto the first cured inner tube and the second cured outer tube.